I thought it a bit strange too. But, the shower comparison is “interesting” anyway.
Various sites say the average US “shower” consumes 17 gallons of water, taking around 8 minutes. That seems a bit high for little kids and the elderly, but, we’ll run with it. I don’t think I get out in 8 minutes sometimes, anyway. :-)
17 gallons = 0.064352000328 cubic meters.
Multiple by 326 million people = 21 million cubic meters.
For comparison, the average flow of the Mississippi River @ Baton Rouge, LA, is 16,800 cu. meters / second. 21 million cu. meters would be, coincidentally, about 21 minutes of Mississippi “flow” @ Baton Rouge. Luckily, we don’t all shower at the same time. (!)
OTOH, this is not a HUGE amount of water. A mere 1000 acre lake averaging 17 ft. deep gets you there.
A better comparison to the mountain collapse, I think, is Mt. Pinatubo’s peak eruption, estimated to have blasted out 5 cubic kilometers of material. That’s 5,000 million cu. meters of material. Some of the pyroclastic flows filled valleys to a depth of 200 meters (660 ft.), with material still at 900 deg. F, five years later.
https://pubs.usgs.gov/fs/1997/fs113-97/
President Putin has a problem, but one not nearly so large as the Filipinos faced!
Another interesting comparison is to a very large building. I found an estimate online of the Willis Tower in Chicago, at 1,798,000 cu. meters. So, call this collapse volume equal to roughly 19 UN-collapsed Willis Towers.
I also found a reference to the World Trade Center at approx. 1,700,000 cu. meters per Tower. So the WTC collapse (both towers, less the adjoining structures) would be almost exactly 1/10 of this mountain. Granted that these structures are obviously (were) not solid.